Interstellar medium phases and abundances in the central parsec: A JWST MIRI/MRS view of the Galactic center

TL;DR

Using JWST/MIRI/MRS 5–27 μm spectroscopy of the Galactic center's circumnuclear disk and central cavity, the study employs CLOUDY-based multiphase modeling to constrain gas temperatures, densities, and abundances. The results reveal a predominantly warm ionized ISM with $T$ between $10^{4}$ and $10^{4.8}$ K, alongside notable molecular gas in the CND and a coronal component at phase interfaces; the CC lacks substantial molecular gas. Abundance analyses show enhanced CNO and α-elements with Fe depletion, implying a chemically young environment driven by recent core-collapse supernovae and stellar winds rather than long-term Type Ia enrichment. Spatial maps indicate a stratified, multiphase ISM with interfaces between molecular, ionized, and coronal gas and a large-scale shock signature in a hot coronal filament near the GC.

Abstract

We used newly obtained observations from the Mid-Infrared Instrument (MIRI) equipped with the Medium Resolution Spectrometer (MRS) aboard the James Webb Space Telescope (JWST) to extract spectra covering the entire spectral range from 5 to 27~$μ$m in the CND and in the CC. We used the photoionization code CLOUDY to generate synthetic spectra with the same spectral range and resolution, simulating a wide range of gas phases and abundances. We then determined the contribution of each phase to the spectra. Once the abundances and contribution from each phase of the gas were determined, we identified four dominant phases and performed a spatial analysis to determine their contribution to each spaxel of the datacubes. We find that in both the CND and the CC, the bulk of the emission originates from warm ionized gas with temperatures of between $10^4$ and $10^{4.8}$~K. In the CND, molecular gas contributes significantly to the flux and is spatially structured, while the CC shows minimal molecular gas content, as is expected from these regions. Coronal gas is detected in both regions at the interface between molecular and warm ionized gas. The observed abundance pattern (enhanced CNO and $α$ elements with suppressed Fe) indicates a chemically young environment, recently enriched by core-collapse supernovae and stellar winds, with a limited contribution from older Type Ia supernovae. This favors a scenario of massive, recent star formation rather than cumulative long-term enrichment.

Figures (17)

• Figure 1: Overview of the GC with the JWST/MIRI-MRS pointings overlaid. The background image is a VLA 2 cm radio map Morris2017 showing the ionized gas structures in the central few parsecs. The cyan rectangle outlines the CC pointing, and the red rectangle shows the footprint of the CND pointing.
• Figure 2: Top: Observed CND spectrum. Middle: Best-fit multiphase model spectrum (red). Bottom: Residuals (observation minus model) as a function of wavelength. The observed spectrum is extracted over the whole CND field of view (extraction and processing in Section \ref{['subsec:data_processing']}); the model is the best-fitting simulation from Section \ref{['subsec:composite_and_abundance']}, using the abundances of Table \ref{['tab:abundances']} and the phase weights of Fig. \ref{['fig:CND_weights']}. Line identifications are shown above the model panel: the most significant lines predicted by the model are marked (see Section \ref{['subsec:line_id']}); when multiple transitions fall within the same spectral bin, only the brightest is labeled here. The complete identification is provided in Table \ref{['tab:line_identificationcnd']}. The y axes of the observation and model spectra are shown on a square-root scale to enhance the visibility of faint features. The two curves are nearly indistinguishable over most of the range; a zoom highlighting fainter features is shown in Fig. \ref{['fig:CND_detailed_comparison_spectra']} (Appendix \ref{['app:appendix_multi']}).
• Figure 3: Same as Fig. \ref{['fig:CND_comparison_spectra']}, but for the CC. The corresponding weights and zoomed-in versions are shown in Figs. \ref{['fig:CC_weights']} and \ref{['fig:CC_detailed_comparison_spectra']}, and the complete identification in Table \ref{['tab:line_identificationcc']}
• Figure 4: Residuals as a function of relative abundances in the CND. Panels show 2D projections for (a) $\alpha$/Fe, (b) CNO/$\alpha$, and (c) CNO/Fe. Darker shades indicate better fits. Diagonal valleys suggest overall enrichment and a relative enhancement of CNO and $\alpha$ elements compared to Fe by $\sim$0.5 dex.
• Figure 5: Same as Fig. \ref{['fig:CND_abundances']}, but for the CC